For many years now, telecommunications carriers have been deploying packet-switched networks in place of, or overlaid upon, circuit-switched networks for reasons of efficiency and economy. Packet-switched networks such as Internet Protocol (IP) or Ethernet networks are intrinsically connectionless in nature and, as a result, suffer from Quality of Service (QoS) problems. Customers value services which are guaranteed in terms of bandwidth and QoS.
One proposal has been to use Ethernet switches in carriers' networks. Use of Ethernet switches in carriers' networks has the advantages of interoperability (mappings between Ethernet and other frame/packet/cell data structures such as IP, Frame Relay and ATM are well known) and economy (Ethernet switches are relatively inexpensive compared to IP routers, for example). Nortel has proposed a form of ‘Connection-oriented Ethernet’ (CoE) known as Provider Backbone Transport (PBT), which is described in WO 2005/099183 and in the paper “Ethernet as Carrier Transport Infrastructure”, David Allan, Nigel Bragg, Alan McGuire, Andy Reid, IEEE Communications Magazine February 2006. PBT is being standardised by the IEEE under the descriptor Provider Backbone Bridging—Traffic Engineering (PBB-TE).
In a conventional Ethernet network switches decide for themselves how to forward packets by processes known as ‘flooding’ and ‘learning’. In a Provider Backbone Transport (PBT) network, these processes are disabled and, instead, managed traffic paths are set up across the network of Ethernet switches. A network manager in the control plane instructs each Ethernet switch along a path to store forwarding instructions. The switch uses the forwarding instructions to forward received data frames. The forwarding instructions operate on a particular combination of identifiers in a data frame, such as a Virtual Local Area Network Identifier (VLAN ID or VID) and the destination MAC address (DA). As traffic is now constrained to follow pre-determined paths through the network, this allows network operators to perform traffic engineering on the Ethernet network, such as planning working and protection paths having diverse routes through the network and provisioning additional trunks to increase capacity.
It is intended that PBT should have a similar range of Operations, Administration and Management (OAM) capabilities as other carrier technologies. One form of OAM is Connectivity Fault Management and this is being standardised in IEEE 802.1ag. IEEE 802.1ag defines that a Continuity Check (CC) message is sent between two endpoints of a trunk at regular intervals, such as every 10 ms. If one end point fails to receive a CC message within a particular time period (e.g. 3.5 times the interval at which CC signals are usually sent) the trunk is declared down, and an Alarm Indication Signal (AIS) may be raised. The failure to receive a CC message can be due to a cable or optical fibre being accidentally severed. In response to the failure condition, a node can switch traffic to a protection path. Typically, a carrier requires this process of switching traffic to a protection path to take place within a period of 50 ms from the fault occurring.
While each CC message, by itself, is a short message requiring a small amount of processing at a node, the accumulated amount of processing required to process regular CC messages on every trunk can present a significant processing burden on a node. Switches can be provisioned with an amount of processing power matched to the demands of the expected CC sessions, but the amount of processing is limited by factors such as cost, space and power consumption/thermal emission of the processors. This can limit the number of CC sessions that can be supported, which in turn limits the number of trunks and the overall size of the network which can be supported. A further disadvantage is that fast CC sessions consume bandwidth. If there is a very large number of fast CC sessions, they will consume a lot of bandwidth, particularly over core links where the CC sessions are likely to be most dense and the bandwidth most expensive. If PBT is applied to national-scale networks and used for point-to-point business services, it is likely that there will be a large number of trunks carrying very few service(s) per trunk.
A similar problem can arise in non-Ethernet networks such as MultiProtocol Label Switching (MPLS) networks.
The present invention seeks to reduce the processing load on network devices to implement OAM sessions, such as Continuity Checks (CC), and/or to reduce the amount of bandwidth occupied by OAM session messaging, such as CC messages.